The present invention relates to video tape recorders of the type provided with two rotating heads and in which television signals are recorded along parallel tracks extending obliquely to the length of the tape.
It is known that in video tape recorders the video signal is recorded by frequency modulating a picture carrier and recording the modulated carrier along oblique tracks which extend obliquely at a predetermined angle, which is about 6.degree. for VHS devices, to the longitudinal direction of the tape. Preferably one picture field, for example, is recorded along each such oblique track. Recording and playback are effected with rotating heads, there being two heads in the VHS system which are alternatingly in contact with the oblique tracks. In this way the relatively high relative speed between head and tape required for video recordings is achieved with a relatively slow tape advance.
In such devices, the audio signal is recorded by a stationary head-on a longitudinal track having a width of about 1 mm and which extends parallel to the edge of the tape. The audio signal is played back by a similar stationary head. Thus, it is the relatively low longitudinal velocity of the tape, and not the substantially high relative speed between the rotating video signal heads and tape on the oblique tracks, which is a major factor in the recording and playback of the audio signal.
Recorders which are presently on the market can operate with values of the order of magnitude of 2 cm/s and less in order to increase the total playing time of the tape. This low relative speed between tape and audio head has a disadvantageous effect on the quality of the recorded audio signal. High fidelity sound quality is practically impossible to achieve with such a low relative speed. The recorded audio signal only has a frequency range of about 70 Hz to 10 kHz. Due to the narrow physical width of the longitudinal track, the associated signal to noise ratio is also relatively poor and becomes even worse if two audio signals are recorded for stereo playback or bilingual playback since the width of each channel is then of necessity half that of the longitudinal track.
It is conceivable to record the audio signal in a manner similar to that employed for video discs on the oblique tracks together with the picture carrier. However, such a solution has not yet been successfully reduced to practice. On the one hand, the available frequency band is already utilized completely. The range from 0-1.3 MHz is taken up by the reduced-frequency, quadrature modulated chrominance subcarrier and the rest of the frequency band is occupied by the frequency spectrum of the modulated picture carrier.
Recording of an audio, or sound, carrier in the remaining narrow frequency gap between the modulated chrominance subcarrier and the frequency spectrum of the picture carrier is practically impossible because of the required steep filter band edges for the chrominance subcarrier and picture carrier due to the occurring phase and group delay errors. On the other hand, if the audio signal is recorded on the oblique tracks there results considerable interference during playback due to the switching between video heads, which does not interfere with picture playback because it takes place during the vertical blanking periods.
During playback of the audio signal, however, this switching of heads becomes audible as an annoying noise component because the playback of the audio signal, which is modulated, for example, on a carrier, is temporarily interrupted due to the switching of heads or is subjected to phase shifts. The resulting spike-type interference pulses at a fundamental frequency of 50 Hz with a large harmonics content produce a continuous, annoying noise during audio playback.